Swath-mapping the Australian continental margin: results from offshore Tasmania

Abstract

Considerable areas of Australia's offshore region were mapped recently using modern multibeam seafloor swath-mapping systems such as Simrad EM12D and Sea Beam 2000. The new and advanced technology built into these systems has led to the acquisition of a vast quantity of new bathymetric data and acoustic backscatter imagery. The detailed structural information available in these data sets and their extensive coverage form the basis for renewed and enlightened examination of the tectonic evolution of parts of the Australian margin. The new data have important implications for petroleum exploration, the assessment and management of fisheries and deep-sea mineral resources, as well as 'Law of the Sea' territorial claims. Almost 200,000 km2 of seafloor were mapped to the west and south of Tasmania in February to March 1994. The survey was conducted by AGSO in co-operation with French scientists using IFREMER's research vessel L'Atalante. The ship is equipped with the Norwegian Simrad EM12D multibeam echosounder. This 162-beam, 13 kHz system mapped bathymetry and seafloor acoustic backscatter ('reflectivity') across a swath up to 20 km wide at a speed of 10 knots (19 km/hr). A total of 13,600 km of underway geophysical data (6-channel seismic, gravity and magnetic) were also collected during this survey. The images of the new survey data over the Tasmanian transform margin and South Tasman Rise are both detailed and spectacular, and provide the first unequivocal insight into the structural development of this region. This development occurred during Late Jurassic to Cretaceous Australia/Antarctica continental extension through both rift and wrench movements, and subsequent post mid-Cretaceous opening of the Southern Ocean. The bathymetric images clearly show the seafloor spreading fabric, over 2 km of relief at the Tasman Fracture Zone, and rotated continental fault blocks rising up to 2.5 km above the abyssal plain west of Tasmania. An extensive system of canyons and a large field of ?Tertiary volcanoes were mapped for the first time on the continental slope. Deep, narrow transtensional basins on the western flank of the South Tasman Rise contain greater than 2.0 s two-way time of sedimentary section.